Combustion in Spark-ignition (SI) engines
Combustion
may be defined as a relatively rapid chemical combination of hydrogen and
carbon in the fuel with the oxygen in the air resulting in the liberation of
energy in the form of heat. Combustion is a very complicated phenomenon and has
been a subject of intensive research for many years.
The conditions necessary for combustion are
(i)
the
presence of a combustible mixture;
(ii)
Some
means of initiation combustion;
(iii)
Stabilization
and propagation of flame in the combustion chamber.
In
the spark-ignition engine or the SI engine a homogeneous carburetted mixture of
petrol vapour and air, in nearly stoichiometric or chemically correct ratio, is
compressed in the compressed in the compression stroke through a small compression
ratio (6:1 to 11:1) and the mixture is ignited at one place before the end of
the compression stroke (say 30 deg before TDC) by means of an electric spark.
After ignition a single definite flame front progresses through the air-fuel
mixture, and entire mixture being in the combustible range.
Stages of combustion
In a spark-ignition engine a sufficiently
homogeneous mixture of vaporized fuel, air and residual gases is ignited by a
single intense and high temperature spark between the spark plug electrodes (at
the moment of discharge the temperature of electrodes exceeds 10000°C), leaving
behind a thin thread of flame. From this thread combustion spreads to the
envelope of mixture immediately surrounding it at a rate which primarily
depends upon the temperature of the flame front itself and to a secondary
degree, upon the temperature and the density of the surrounding envelope.
In
this manner there grows up, gradually at first, a small hollow nucleus of
flame, much in the manner of a soap bubble. If the contents of the cylinder
were at rest, this flame bubble would expand with steadily increasing speed
until extended throughout the whole mass. In the actual engine cylinder,
however, the mixture is not at rest. It is in fact, in a highly turbulent condition.
The turbulence breaks the filament of flame into a ragged front, thus
presenting a far greater surface area from which heat is radiated; hence its
advance is speeded up enormously. The rate at which the flame front travels is
dependent primarily on the degree of turbulence, but its general direction of
movement, that of radiating outward from the ignition point is not much
affected.
The
combustion in spark ignition engine can be imagined as if developing in two
stages. One- the growth and development of a semi propagating nucleus of flame
called the ignition lag or preparation phase, and the other, the spread of the
flame throughout the combustion chamber.
